Bring back to_symbolic to allow MTK to work without change

This reverts commit a725bbb.

Author: shashi

src/api.jl

@@ -26,7 +26,7 @@ function simplify(x;
         Fixpoint(rewriter)
     end
 
-    PassThrough(f)(x)
+    PassThrough(f)(to_symbolic(x))
 end
 
 Base.@deprecate simplify(x, ctx; kwargs...)  simplify(x; rewriter=ctx, kwargs...)
@@ -40,8 +40,8 @@ the corresponding value.
 function substitute(expr, dict; fold=true)
     rs = Prewalk(PassThrough(@rule ~x::(x->haskey(dict, x)) => dict[~x]))
     if fold
-        rs(expr) |> SymbolicUtils.fold
+        rs(to_symbolic(expr)) |> SymbolicUtils.fold
     else
-        rs(expr)
+        rs(to_symbolic(expr))
     end
 end

src/types.jl

@@ -67,8 +67,21 @@ Base.isequal(x::Symbolic, y::Symbolic) = false
 Convert `x` to a `Symbolic` type, using the `istree`, `operation`, `arguments`,
 and optionally `symtype` if available.
 """
-function to_symbolic end
-Base.@deprecate to_symbolic(x) x
+to_symbolic(x::Symbolic) = x
+
+function to_symbolic(x)
+    if !istree(x)
+        return x
+    end
+
+    op = to_symbolic(operation(x))
+
+    if symtype(x) === Any
+        Term(op, map(to_symbolic, arguments(x)))
+    else
+        Term{symtype(x)}(op, map(to_symbolic, arguments(x)))
+    end
+end
 
 Base.one( s::Symbolic) = one( symtype(s))
 

test/interface.jl

@@ -15,9 +15,10 @@ end
 
 ex = 1 + (:x - 2)
 
-@test simplify(ex) == ex # Not simplified because symtype Any
+SymbolicUtils.to_symbolic(ex::Expr) = ex
 
-SymbolicUtils.symtype(::Expr) = Real
+@test simplify(ex) == ex
 
+SymbolicUtils.symtype(::Expr) = Real
 @test simplify(ex) == -1 + :x
 @test simplify(:a * (:b + -1 * :c) + -1 * (:b * :a + -1 * :c * :a), polynorm=true) == 0